1. Field of the Invention
This invention relates to a high frequency amplifying apparatus for a television tuner or a CATV converter.
2. Description of the Prior Art
A high frequency amplifying apparatus is known which comprises a broad-band amplifier and an AGC (automatic gain control) circuit and is used in a television tuner, a CATV converter, a BS tuner, or the like.
Hereinbelow will be described such a prior art high frequency amplifying apparatus. FIG. 2 is a block diagram of a portion of a television tuner including a prior art high frequency amplifying apparatus 50b. In FIG. 2, numeral 1 is an input filter, numeral 2 is an AGC circuit, numeral 3 is an RF amplifying circuit (hereinafter referred to as an RF amplifier), numeral 4b is another AGC circuit, and numeral 5 is a mixer.
FIG. 3 is a schematic circuit diagram of the prior art high frequency amplifying apparatus 50b. In FIG. 3, numeral 6 is an RF (radio frequency) amplifier for RF amplifying which may comprise a discrete transistor circuit or an integrated circuit. Numerals 107 and 108 are AGC drive transistors, numerals 9, 10, and 11 are coupling capacitors, numeral 112 is a bypass capacitor, numerals 113 to 120 are resistors, numeral 21, 23, and 122 are choke coils, and numeral 124 and 125 are pin diodes.
Hereinbelow will be described operation of the prior art high frequency amplifying apparatus having the structure mentioned above. In FIG. 2, a high frequency signal inputted from a terminal A is filtered by the input filter 1. An output of the input filter 1 is applied to the AGC circuit 2. An output signal of the AGC circuit 2 whose gain is controlled is amplified by the RF amplifier 3. An output of the RF amplifier 3 is sent to another AGC circuit 4. The reason why the high frequency signal is passed through two AGC circuits 2 and 4 is to obtain a sufficient gain control range and to prevent a decrease in a noise figure. The gain-controlled signal is applied to a mixer 5 for converting the high frequency signal into an intermediate frequency signal. The converted intermediate frequency signal is outputted at a terminal B.
On the other hand, in FIG. 3, the high frequency signal inputted from a terminal C is passed through a coupling capacitor 9 and an RF amplifier 6. A load of the RF amplifier 3 is a choke coil because the RF amplifier 3 is a broad-bandpass type one. The output of amplifier 3 is applied to an AGC circuit 4b through the coupling capacitor 10. An AGC voltage is inputted from a terminal E to the transistor 107 provided for current amplification. The amplified AGC signal passes the choke coil 23 and the pin diode 124, and flows into the ground passing though a resistor 120 and a choke coil 122. The AGC voltage at the terminal E for the maximum gain of the AGC circuit 4 is about 7 to 8 V. A voltage of the AGC signal at an anode of the diode 124 is lower than this voltage by a voltage difference between two diodes. On the other hand, a voltage is supplied to an anode of the diode 125 from a transistor 108 in order to set a voltage of the anode of the diode 125. When the gain is set to the maximum, the diode 125 is turned off. Biasing of the transistor 108 is so determined that the diode 125 conducts gradually with decrease in the voltage at the terminal E for attenuation of the gain. The gain-controlled signal is outputted at a terminal D through the coupling capacitor 11. It is necessary to flow a current more than 10 mA through the pin diode to turn on it and the choke coil 23 is necessary to stop a high frequency signal and cannot be replaced with a resistor.
In the prior art having the structure mentioned above, there are problems that the circuit is complicated and there are a lot of parts. Particularly, there are a lot of choke coils in the circuit.